Metro and long-haul transmission at high data rate is critical to the substantial expansion of broadband services, especially with the growth of services such as YouTube and BBC iPlayer. However, as the capacity of optical transmission systems grows, optical impairments resulting in signal degradation become prominent and require careful compensation. Among them, chromatic dispersion (CD) is the most important impairment which can limit the transmission reach to less than 100 km for 10 Gbit/s OOK signal and 6 km at 40 Gbit/s. The commercial optical networks employ in-line dispersion compensation fibre (DCF), which is bulky and expensive with significant power attenuation. It's length has to be manually adjusted to achieve proper CD compensation with the result that link provisioning is expensive and time consumption.
Electronic dispersion compensation (EDC) has attracted much interest recently for extending reach in legacy multimode optical fiber as well as in metro and long-haul optical transmission systems. Its advantages compared to the optical compensation method include:                Reduced costs by eliminating the need for DCF modules including the cost of DCFs and the associated cost for compensating the loss from the DCFs.        Simplification of the deployment and configuration        Flexible and adaptive compensation required in dynamic optical networks.        Easy for integration in transmitter and receiverEDC can be categorized into transmitter-side EDC and receiver-side EDC. Transmitter-side EDC, as shown in FIG. 1, generates pre-distorted optical pulse shape for signal modulation according to the amount of dispersion to be compensated. In such a technique, the pre-distortion parameters are preset according to the information given by the CD for each link. However, the optimum setting often changes due to different routing paths and waveform changes arising from fibre nonlinearity, transmitter chirps etc. Because the errors are detected at the receiver, the feedback time for adaptive compensation is unacceptable due to the round-trip time. Transmitter side EDC is disclosed in more detail in U.S. Pat. No. 7,382,984, McNicol et al and in a paper publication entitled “Electronic dispersion compensation”, in Proc. Optical Fiber Communication Conference (OFC) 2006, paper OWK1. McGhan et al.        
In contrast, receiver-side EDC, which can adapt quickly to changes in link conditions, is of particular value for future transparent optical networks where the re-configurability of the add-nodes and drop-nodes causes the transmission paths to vary. It can also be used for compensating polarization mode dispersion (PMD). PMD is an impairment which would severely degrade system performance for transmission distance beyond several hundred kilometers at 10 Gbit/s. Different from CD, PMD which occurs as a result of birefringence in optical fibre depends on stress and vibration as well as random changes in the state of light polarization, and may dynamically change as time in a scale of milliseconds (ms). Up to now, 40 Gbit/s feed forward equalizer (FFE) and decision feedback equalizer (DFE), and 10 Gbit/s maximum likelihood sequence estimation (MLSE) have been fabricated, for example as shown in U.S. Pat. No. 7,184,478, Popescu et al, or a paper published by M. Nakamura, K. Murata, and M. Tokumitsu, “Advances in 40 G electronic equalizers”, Proc. Optical Fiber Communication Conference (OFC) 2007, paper OThN6, or a paper published by H. Jiang, and R. Saunders, “Advances in SiGe ICs for 40 Gb/s signal equalizers”, Proc. Optical Fiber Communication Conference (OFC) 2006, paper OTuE1, or a paper published by A. Farbert, S. Langenbach, N. Stojanovic, C. Dorschky, T. Kupfer, C. Schulien, J.-P. Elbers, H. Wernz, H. Griesser, and C. Glingener, “Performance of a 10.7 Gb/s receiver with digital equaliser using maximum likelihood sequence estimation”, European Conference on Optical Communication (ECOC), Post-deadline Th4.1.5, 2004.
However, these conventional EDCs are based on direct detection (DD EDC) and their performance is limited due to loss of the signal phase information. In addition, the transformation of linear optical impairments into nonlinear impairments after square-law detection significantly increases the operational complexity of the DD EDC. For example, DD MLSE, regarded as an optimized DD EDC as shown in FIG. 2, can extend the transmission reach to ˜200 km at 4 states. It has been theoretically predicted that it can achieve 700 km SMF transmission but at the problem of increased electronic processing complexity requiring 8192 Viterbi processor states, as disclosed in a published paper by G. Bosco and P. Poggiolini, “Long-distance effectiveness of MLSE IMDD receivers”, IEEE Photo. Technol. Lett. 18, 1037-1039 (2006).
EDC based on coherent detection can access both the intensity and the phase, thus exhibiting better performance compared to DD EDC, as shown in U.S. Pat. No. 7,266,310, Savory et al and a paper published by M. G. Taylor, “Coherent detection for optical communications using digital signal processing”, in Proc. Optical Fiber Communication Conference (OFC) 2007, paper OMP1. However, these systems require a complicated optical implementation due to phase and polarization tracking between the local oscillator and the signal, as shown in FIG. 3.
Recently, a novel cost-effective EDC receiver was proposed, which accesses the intensity and instantaneous frequency information simultaneously, using a single asymmetric Mach-Zehnder interferometer (AMZI). The overall idea was covered by the previous PCT Patent Publication number WO2006131904, assigned to the assignee of the present invention, as shown in FIG. 4 and FIG. 5. Compared to DD EDC where only optical intensity information is available, this method exhibits better compensation performance due to the recovered phase information. It also features better cost effectiveness compared to coherent detection based EDC by avoiding complicated polarization and phase tracking.
However, the two-input one-output electrical signal processing module is required. It is clear that proper design of the electrical signal processing module for impairment compensation is essential to take full use of the extracted optical information.
Previously, the recovered intensity and instantaneous frequency information was used to reconstruct the full optical field, which allows for full-field CD compensation in the electrical domain using a fixed dispersive transmission line, as shown in FIG. 6. Owing to the benefits from the recovery of the knowledge of phase information, full-field EDC using a dispersive transmission line was experimentally demonstrated to recover a 10 Gbit/s OOK signal after transmission through a 500 km of BT Ireland's field-installed SMF between Tyndall National Institute, Cork City, Ireland and Clonakilty, Co. Cork, Ireland without using either in-line compensation or coherent detection, as disclosed in a paper J. Zhao, M. E. McCarthy, P. Gunning, A. D. Ellis, “Mitigation of pattern sensitivity in full-field electronic dispersion compensation”, IEEE Photo. Technol. Lett., 21, 48-50, 2009. FIG. 6 shows the experimental setup of technology demonstration. For commercial applications, the real-time oscilloscope can be replaced by ADCs while the signal processing module described in the dotted box is implemented using analogue devices or digital devices such as application specific integrated circuit (ASIC) or field programmable gate array (FPGA). Note that in this technique (FIG. 6), the local oscillator is employed to re-construct the full field in the electrical domain after the optical signals have been optically processed by asymmetric Mach-Zehnder interferometer and optical-to-electrical converted by photodiodes. This is evidently different from the local oscillator used in FIG. 3 which is a narrow-linewidth laser and is used for detecting the optical signals. Electrical oscillator features cost-effective, well-designed specifications, and easily-controlled phase when compared to an optical laser.
Although the dispersive transmission line method is cost effective, a problem is that it has to be matched to particular CD encountered in the transmission and is inflexible for adaptive CD compensation in transparent optical networks. If the transmission paths change because of path re-routing or a path failure, a different dispersive transmission line must be used. Furthermore, dispersive transmission line has no capability for compensation of PMD, which would dynamically change and play an important role for transmission beyond 500 km. As a result, dispersive transmission line may not extend the transmission reach to thousand kilometers even when CD can be fully compensated. In addition, dispersive transmission line cannot mitigate the impairment arising from filtering at the transmitter, add- and drop-nodes, and receiver, e.g. narrow-band filtering by means of WDM multiplexers and de-multiplexers. Narrow band filtering is essential in wavelength-division multiplexed (WDM) optical transmission systems to avoid WDM crosstalk and achieve high spectral efficiency. In addition, the performance of the dispersive transmission line is sensitive to non-optimized system parameters and a large penalty would be induced when the system parameters are not ideal (that is applicable for practical systems). Our recent experiment shows that this method cannot be used to extend the transmission reach beyond 500 km unless system devices with very tight specifications are used. Furthermore, in digitally implemented dispersive transmission line, higher sampling rate analogue-to-digital converters with 4 or more samples per symbol should be used to avoid large performance penalty, which inevitably weakens the advantage of cost-effectiveness of this method. Finally, the dispersive transmission line method lacks automatic optimization capability for system parameters or the ability to equalize the impairments arising from improper setting of these parameters. For example, in the previous experiments, the scaling factor for compensating gain imbalance between the detected two signals, the bias added to the intensity, sampling phase etc were manually optimized, which was time consuming and non-optimal. Since some of these parameters vary as time (e.g. sampling phase) and some are changed when the CD value changes or the path is re-routed (e.g. scaling factor), self-adaptive optimization for these parameters is highly desirable.
Other methods have been proposed to enable adaptive dispersion compensation, as disclosed in J. Zhao et al “Chromatic dispersion compensation using full optical-field maximum likelihood sequence estimation”, Optical Fiber Communication Conference OThE6, 2009, and US patent publication number US2006/013597A1 D. E. Crivelli et al “Compensating impairments of optical channel using adaptive equalization”. The technique disclosed in US2006/013597A1 employs an adaptive equalizer following an optical receiver front end. The optical receiver front end uses an optical local oscillator (laser), or coherent detection as discussed previously, to detect the signals. Coherent detection is different from the direct-detection based full-field detection. It does not need full-field reconstruction circuit in the electrical domain, but requires much higher implementation complexity in the optical domain, including narrow-linewidth optical laser, 90 degree hybrid etc, and additional polarization and phase stabilization. The method disclosed in J. Zhao et al, “Chromatic dispersion compensation using full optical-field maximum likelihood sequence estimation” also enables adaptive dispersion compensation by employing direct-detection based full-field detection. However, this method has limited capability for long-distance transmission as the complexity of the maximum likelihood sequence estimation (MLSE) increases exponentially with the compensated dispersion value. For example, the method requires four Viterbi processor states to achieve 372 km transmission, sixteen Viterbi states for 500 km, and impractically more than one hundred Viterbi states for 700 km at 10 Gbit/s. Consequently, most of the commercial adaptive electronic compensation products (direct-detection based products, and no full-field detection based products to the best of our knowledge) only have the compensation capability of less than 300 km at 10 Gbit/s. In addition, the performance of maximum likelihood sequence estimation is optimal only when the noise statistics of samples into the MLSE is uncorrelated, or whitened, which, however, is not true for practical optical front end. Finally, in addition to the exponentially increasing complexity of the equalization processor, a fast-adaptive MLSE also requires an adaptive algorithm and associated channel estimation circuit. This algorithm and circuit significantly increase additional implementation complexity, which also exponentially increases with the intersymbol-interference spans. A large-state MLSE also requires a longer self-adaptive time to track the changes in the network configurations and consequently degrades the performance for fast adaptation. For example, it would require a much longer training sequence to acquire the channel estimation for a 100-state maximum likelihood sequence estimation than that using only 4 states.
In addition to the complexity and degraded adaptation speed of MLSE for long-distance transmissions, the technique is usually implemented in the digital domain and it is essential to synchronize the symbol rate of the transmitted signal and the rate of the sampled digital signal. However, if the signal is highly distorted, e.g. after 500 km transmission without dispersion compensation, the eye has been completely closed and timing recovery is difficult.
Presently, there is no technical solution to solve all the above mentioned problems. For example, realizing an ideal optical front-end for optimal operation condition of full-field MLSE, and using full-field MLSE for a longer-distance transmission are impractical due to exponentially increase complexity. A MLSE for long-distance transmission also has a degraded adaptation speed accordingly and increases the difficulty for symbol synchronization. Dispersive transmission line has a poorer tolerance and large penalty to non-optimized system parameters. It can also not be used to extend the transmission reach beyond 500 km at 10 Gbit/s unless system devices with very tight specifications are used. Higher sampling rate is required in digitally implemented dispersive transmission line to avoid large performance penalty inevitably increases the implementation complexity, destroying the advantages of cost-effectiveness of the dispersive transmission line method. Dispersive transmission line also lacks adaptive compensation capability.
The optimal design of a practical EDC system require not only the understanding of the features of the methods in terms of their operation complexity and performance conditions, but also how their advantages can be complementary each other such that their performance drawbacks can be overcome to achieve an optimal solution for applications while the operation complexities are not simply increased on a add basis. For example, the required sampling rate of digitally implemented dispersive transmission line can be reduced at a tolerable penalty and its performance sensitivity to non-optimized system parameters can be reduced, when combined with a certain method. Full-field MLSE can approach the optimal condition, increase the adaptation speed, and reduce the complexity under the help of a certain method to whiten the noise and reduce the required state number.
An optimal design also requires the identification of features in practical applications. The impairments such as polarization mode dispersion can change quickly with time (μs), but this impairment usually has a short inter-symbol interference span. The non-ideal impulse responses from for example optical and electrical filtering at the transmitter or receiver would break the optimal operation conditions of the MLSE but they are usually static or slow-varying (e.g. in a scale of s). Chromatic dispersion can change quickly in dynamic packet-switched optical networks (μs), but it usually has a dispersion range, e.g. 1000 ps/nm-2000 ps/nm, in which the adaptive compensation range is 1000 ps/nm. Adaptive compensation of all inter-symbol interference spans using an adaptive equalizer regardless of the static or adaptive feature of the impairment sources, will not only result in non-optimized performance as discussed previously, but also increase the complexity of the equalization processor and adaptive algorithm for channel estimation. Simply trying to use an adaptive equalizer for all intersymbol interference spans also requires a longer training sequence and reduces the adaptation speed.
There is therefore a need to provide a system and method for overcoming the above mentioned problems.